New CP Violation in Neutrino Oscillations

CERN-TH/2001-125 IFIC/01-23 WIS/9/01-May-DPP hep-ph/0105159

New CP Violation in Neutrino Oscillations
M. C. Gonzalez-Garcia1,2,3 ? , Y. Grossman4 ? , A. Gusso1,5
1 Instituto ?

and Y. Nir6

§

arXiv:hep-ph/0105159v2 6 Jan 2002

de F? ?sica Corpuscular, Universitat de Val` encia – C.S.I.C Edi?cio Institutos de Paterna, Apt 22085, 46071 Val` encia, Spain 2 Theory Division, CERN CH1211, Geneva 23, Switzerland 3 C.N. Yang Institute for Theoretical Physics State University of New York at Stony Brook Stony Broow,NY 11794-3840, USA 4 Department of Physics, Technion–Israel Institute of Technology Technion City, 32000 Haifa, Israel 5 Instituto de F? ?sica Te? orica, Universidade Estadual Paulista Rua Pamplona 145, 01405–900, S? ao Paulo, Brazil 6 Department of Particle Physics, Weizmann Institute of Science Rehovot 76100, Israel

Abstract
Measurements of CP–violating observables in neutrino oscillation experiments have been studied in the literature as a way to determine the CP– violating phase in the mixing matrix for leptons. Here we show that such observables also probe new neutrino interactions in the production or detection processes. Genuine CP violation and fake CP violation due to matter e?ects are sensitive to the imaginary and real parts of new couplings. The dependence of the CP asymmetry on source–detector distance is di?erent from the standard one and, in particular, enhanced at short distances. We estimate that future neutrino factories will be able to probe in this way new interactions that are up to four orders of magnitude weaker than the weak interactions. We discuss the possible implications for models of new physics.

? concha@thwgs.cern.ch ? yuvalg@physics.technion.ac.il ? gusso@i?c.uv.es § ftnir@wicc.weizmann.ac.il

0

I. NEW CP VIOLATION IN NEUTRINO INTERACTIONS

In the future, neutrino oscillation experiments will search for CP–violating e?ects [1–22]. The Standard Model, extended to include masses for the light, active neutrinos, predicts that CP is violated in neutrino oscillations through a single phase in the mixing matrix for leptons. This e?ect is suppressed by small mixing angles and small mass di?erences. It is not unlikely, however, that the high-energy physics that is responsible for neutrino masses and mixing involves also new neutrino interactions. Such interactions provide new sources of CP violation. In this work we study CP–violating e?ects due to contributions from new neutrino interactions to the production and/or detection processes in neutrino oscillation experiments. We investigate the following questions: (i) How would new, CP–violating neutrino interactions manifest themselves in neutrino oscillations? (ii) Are the e?ects qualitatively di?erent from the Standard Models ones? In particular, can we use the time (or, equivalently, distance) dependence of the transition probability to distinguish between Standard Model and new CP violation? (iii) How large can the e?ects be? In particular, do the new interactions su?er from suppression factors related to mixing angles and mass di?erences? (iv) Can the new CP violation be observed in proposed experiments? What would be the optimal setting for these observations? (v) Which models of New Physics can be probed in this way? The plan of this paper goes as follows. In section II we present a parameterization of the New Physics e?ects that are of interest to us and explain the counting of independent CP–violating phases in our framework. In section III we evaluate the New Physics e?ects on the transition probability in neutrino vacuum oscillation experiments. (A full expression for the transition probability, without any approximations concerning mixing angles and mass di?erences, is given in Appendix A.) In section IV we investigate the resulting CP asymmetry and compare the New Physics contribution to the standard one (that is, the contribution to the asymmetry from lepton mixing). In sections V and VI we evaluate the New Physics e?ects on, respectively, the transition probability and CP asymmetry, in neutrino matter oscillations. In section VII we study how these e?ects can be observed in future neutrino factory experiments. In particular, we estimate a lower bound on the strength of the new interactions that can be observed in these experiments. This lower bound is compared to existing model–independent upper bounds in section VIII. We summarize our results and discuss some of the implications that would arise if a signal is experimentally observed in section IX.
II. NOTATIONS AND FORMALISM

In this section we give a model–independent parameterization of New Physics e?ects on production and detection processes in neutrino oscillation experiments. We put special emphasis on CP–violating phases. We denote by |νi , i = 1, 2, 3, the three neutrino mass eigenstates. We denote by |να the weak interaction partners of the charged lepton mass eigenstates α? (α = e, ?, τ ):

1

|να =

i

Uαi |νi .

(2.1)

Whenever we use an explicit parameterization of the lepton mixing matrix [23,24], we will use the most conventional one: U ≡ U23 U13 U12 1 0 ? ≡ ? 0 c23 0 ?s23
?

c13 0 ?? 0 s23 ? ? ?s13 e?iδ c23

??

c12 0 s13 eiδ ?? 1 0 ? ? ?s12 0 0 c13
??

s12 c12 0

0 ? 0?, 1

?

(2.2)

with sij ≡ sin θij and cij ≡ cos θij . Alternatively, a convention-independent de?nition of the phase δ that we will use in our calculations is given by δ ≡ arg
? Ue3 U? 3 . ? Ue1 U?1

(2.3)

We consider new, possibly CP–violating, physics in the production and/or detection process. Such e?ects were previously studied in Ref. [25], and we follow closely the formalism of that paper. Most of the analysis in [25], however, was carried out assuming CP conservation. We parameterize the New Physics interaction in the source and in the detector by two sets of d s e?ective four–fermion couplings, (Gs NP )αβ and (GNP )αβ , where α, β = e, ?, τ . Here (GNP )αβ refers to processes in the source where a νβ is produced in conjunction with an incoming α? or an outgoing α+ charged lepton, while (Gd NP )αβ refers to processes in the detector where an incoming νβ produces an α? charged lepton. While the SU (2)L gauge symmetry requires that the four–fermion couplings of the charged current weak interactions be proportional to GF δαβ , new interactions allow couplings with α = β . Phenomenological constraints imply that the new interaction is suppressed with respect to the weak interaction, | (G s NP )αβ | ? GF , | (G d NP )αβ | ? GF . (2.4)

For the sake of concreteness, we consider the production and detection processes that are relevant to neutrino factories. We therefore study an appearance experiment where neutrinos are produced in the process ?+ → e+ να ν ?? and detected by the process νβ d → ?? u, and antineutrinos are produced and detected by the corresponding charge-conjugate processes. Our results can be modi?ed to any other neutrino oscillation experiment in a straightforward d way. The relevant couplings are then (Gs NP )eβ and (GNP )?β . It is convenient to de?ne small dimensionless quantities ?s,d αβ in the following way: ?s eβ ≡ ?d ?β ≡ (G s NP )eβ
s s 2 2 2 |GF + (Gs NP )ee | + |(GNP )e? | + |(GNP )eτ |

, . (2.5)

(G d NP )?β

d d 2 2 2 |GF + (Gd NP )?? | + |(GNP )?e | + |(GNP )?τ |

Since we assume that |?s,d αβ | ? 1, we will only evaluate their e?ects to leading (linear) order. New ?avor–conserving interactions a?ect neutrino oscillations only at O(|?|2 ) and will be neglected from here on. (More precisely, the leading e?ects from ?avor–diagonal 2

couplings are proportional to ? (?avor–diagonal)×?(?avor–changing) and can therefore be safely neglected.) We use an explicit parameterization for only two of the ?’s, with the following convention:
s iδ? ?s e? ≡ |?e? |e , ? d? iδ? ?d ?e ≡ |??e |e .


(2.6)

′ Alternatively, we can de?ne the phases δ? and δ? in a convention-independent way:

δ? ≡ arg

?s e? , ? Ue1 U? 1

′ δ? ≡ arg

? ?d ?e . ? Ue1 U? 1

(2.7)

We would like to conclude this section with a comment on the number of independent CP–violating phases in our framework. It is well known that the three–generation mixing matrix for leptons depends, in the case of Majorana neutrinos, on three phases. Two of these, related to the fact that there is no freedom in rede?ning the phases of neutrino ?elds, do not a?ect neutrino oscillations and are therefore irrelevant to our discussion. The other one is analogous to the Kobayashi–Maskawa phase of the mixing matrix for quarks. The freedom of rede?ning the phases of charged lepton ?elds is fully used to reduce the number of relevant phases to one. Consequently, it is impossible to remove any phases from the ?s,d αβ parameters. Each of these parameters introduces a new, independent CP–violating phase. d For example, when we discuss νe → ν? oscillations, our results will depend on ?s e? , ??e ? and the Uei U?i (i = 1, 2, 3) mixing parameters. This set of parameters depends on three independent phases, one of which is the δ of Eq. (2.3), while the other two can be chosen ′ to be δ? and δ? of Eq. (2.7). This situation is illustrated in Fig. 1, where we show in the complex plane the unitarity triangle and the ?s,d parameters that are most relevant to νe → ν? oscillations.
III. THE TRANSITION PROBABILITY IN VACUUM

In this section we derive the expression for the transition probability in neutrino oscillation experiments as a function of the mixing matrix parameters and the New Physics parameters. s We denote by νe the neutrino state that is produced in the source in conjunction with + d an e , and by ν? the neutrino state that is signalled by ?? production in the detector:
s |νe = d |ν? s Uei + ?s e? U?i + ?eτ Uτ i |νi , d U?i + ?d ?e Uei + ??τ Uτ i |νi .

i

=
i

(3.1)

(Note that the norm of the states so de?ned is one up to e?ects of O(|?|2 ), which we consistently neglect.) We obtain the following expression for the transition probability Pe? = d s s s | ν? |νe (t) |2 , where νe (t) is the time-evolved state that was purely νe at time t = 0 :
2

Pe? =
i

e

?iEi t

? Uei U?i

+

2 ?s e? |U?i |

+

2 ? ?d ?e |Uei |

+

? ?s eτ Uτ i U?i

+

? ? ?d ?τ U?i Uτ i

.

(3.2)

3

Our results will be given in terms of ?m2 ij , ?ij and xij , which are de?ned as follows:
2 2 ?m2 ij ≡ mi ? mj ,

?ij ≡ ?m2 ij /(2E ),

xij ≡ ?ij L/2,

(3.3)

where E is the neutrino energy and L is the distance between the source and the detector. Equation (3.2) will be the starting point of our calculations. The full expression for Pe? in vacuum is given in Appendix A and has been used for our numerical calculations described below. To understand the essential features of our analysis it is, however, more SM useful to do the following. First, we separate Pe? into a Standard Model piece, Pe? , and a s,d NP SM New Physics piece, Pe? . What we mean by Pe? is Pe? (?αβ = 0). This is the contribution to Pe? from the Standard Model extended to include neutrino masses but no new interactions. NP In contrast, Pe? contains all the ?s,d αβ –dependent terms. Second, since the atmospheric and reactor neutrino data imply that |Ue3 | is small and the solar neutrino data imply that 2 SM NP ?m2 12 /?m13 is small, we expand Pe? to second order and Pe? to ?rst order in |Ue3 | and 2 ?m12 . SM For Pe? we obtain:
SM 2 2 2 2 2 Pe? = 4x2 21 |Ue2 | |U?2 | + 4 sin x31 |Ue3 | |U?3 | ? ? + 4x21 sin 2x31 Re(Ue2 Ue 3 U?2 U?3 ) ? ? ? 8x21 sin2 x31 I m(Ue2 Ue 3 U?2 U?3 ).

(3.4)

The ?rst term is the well known transition probability in the two–generation case. The second term gives the well known transition probability in the approximation that ?m2 12 = 0. The last term is a manifestation of the Standard Model CP violation. NP For Pe? we obtain
NP ? d? s 2 s ? Pe? = ? 4 sin2 x31 Re Ue 3 U?3 ??e + ?e? (1 ? 2|U?3 | ) ? 2?eτ U?3 Uτ 3 ? s 2 s ? + 4x21 sin 2x31 Re Ue 2 U?2 ?e? |U?3 | + ?eτ U?3 Uτ 3 ? d? s 2 s ? ? 4x21 I m Ue 2 U?2 ??e + ?e? (1 ? |U?3 | ) ? ?eτ U?3 Uτ 3 ? d? s ? 2 sin 2x31 I m Ue 3 U?3 (??e + ?e? ) ? 2 s ? s ? 4x21 cos 2x31 I m Ue 2 U?2 ?e? |U?3 | + ?eτ U?3 Uτ 3 .

(3.5)

The last three terms in this expression are CP–violating and would be the basis for our results.
IV. CP VIOLATION IN VACUUM OSCILLATIONS

To measure CP violation, one will need to compare the transition probability Pe? evaluated in the previous section to that of the CP-conjugate process, Pe ?? ? . The latter will be measured in oscillation experiments where antineutrinos are produced in the source in

4

conjunction with e? and detected through ?+ production. It is clear that a CP transforma?α′ . As concerns the tion relates the production processes, ?? → e? ν ?α να′ and ?+ → e+ να ν + ? detection processes, ν ?β u → ? d and νβ d → ? u, the situation is less straightforward. We d ? ? + + ? have Gβ? ∝ p? |? ? u ? νβ d|νβ n and Gd ? dν ?β u|ν ?β p . The relation is through ?? β ? ∝ n? |? CP and crossing symmetry, but for a four-fermion interaction this is equivalent to a CP transformation. CP transformation of the Lagrangian takes the elements of the mixing matrix and the ?-terms into their complex conjugates. It is then straightforward to obtain the transition probability for antineutrino oscillations. Our interest lies in the CP asymmetry, ACP = where P± = Pe? ± Pe ?? ?. (4.2) P? , P+ (4.1)

We quote below the leading contributions for ‘short’ distances, x31 ? 1. In some of the observables, we consider two limiting cases for |Ue3 |: The “large” s13 limit : The small s13 limit : x21 /x31 ? |(Ue3 U?3 )/(Ue2 U?2 )|, x21 /x31 ? |(Ue3 U?3 )/(Ue2 U?2 )|. (4.3)

The CP conserving rate P+ is always dominated by the Standard Model. It is given by P+ =
2 8x2 31 |Ue3 U?3 | 2 8x2 21 |Ue2 U?2 |

large s13 , small s13 .

(4.4)

The CP–violating di?erence between the transition probabilities within the Standard Model can be obtained from Eq. (3.4):
SM ? ? P? = ?16x21 x2 31 I m(Ue2 U?2 Ue3 U?3 ).

(4.5)

As is well known, CP violation within the Standard Model is suppressed by both the small mixing angle |Ue3 | and the small mass-squared di?erence ?m2 12 . More generally, it is propor? ? tional to the Jarlskog measure of CP violation, J = I m(Ue2 U? 2 Ue3 U?3 ). For short distances SM (x21 , x31 ? 1), the dependence of P? on the distance is L3 . Since it is CP–violating, it should be odd in L. The absence of a term linear in L comes from the fact that the Standard Model requires for CP to be violated, that all three mass-squared di?erences do not vanish, that is, P? ∝ ?21 ?31 ?32 . In the limit x21 /x31 ? |(Ue3 U?3 )/(Ue2 U?2 )|, we obtain the following Standard Model asymmetry: ASM CP
? Ue2 U? 2 . = ?2x21 I m ? Ue3 U?3

(4.6)

In the small s13 limit, the standard CP violation is unobservably small. The CP–violating di?erence between the transition probabilities that arises from the New Physics interactions can be obtained from Eq. (3.5): 5

NP P? =

? d? s ?8x31 I m[Ue 3 U?3 (??e + ?e? )] large s13 , ? d? s ?8x21 I m[Ue 2 U?2 (??e + ?e? )] small s13 .

(4.7)

We learn that CP violation beyond the weak interactions requires only that either |Ue3 | or ?m2 21 be di?erent from zero, but not necessarily both. Also the dependence on the distance NP is di?erent: for short distances, P? ∝ L. From Eqs. (4.4) and (4.7) we obtain the following new physics contribution to the CP asymmetry: ANP CP =
? ? ? ??
1 Im x31
? s ?d ?e +?e? ? Ue3 U?3 ? s ?d ?e +?e? ? Ue2 U? 2

large s13 , (4.8) small s13 .

The apparent divergence of ANP CP for small L is only due to the approximations that we used. Speci?cally, there is an O(|?|2 ) contribution to P+ that is constant in L [25], namely P+ = O(|?|2 ) for L → 0. In contrast, P? = 0 in the L → 0 limit to all orders in |?|. Equations (4.7) and (4.8) lead to several interesting conclusions: (i) It is possible that, in CP–violating observables, the New Physics contributions compete with or even dominate over the Standard Model ones in spite of the superweakness of the interactions (|?| ? 1). Given that for the proposed experiments x31 < ? 1, it is su?cient that
d max |?s e? |, |??e | ≥ min (|Ue3 |, x21 ) ,

? ? ? ? x1

Im 21

(4.9)

for the new contribution to the CP–violating di?erence P? to be larger than the standard one. NP SM (ii) The di?erent distance dependence of P? and P? will allow, in principle, an unambiguous distinction to be made between New Physics contributions of the type described here and the contribution from lepton mixing. (iii) The 1/L dependence of ANP CP suggests that the optimal baseline to observe CP violation from New Physics is shorter than the one optimized for the Standard Model. We carried out a numerical calculation of the probabilities P± and asymmetry ACP as a function of the distance between the source and the detector. We use Eν = 20 GeV, which is the range of neutrino energy expected in neutrino factories. For the neutrino parameters, ?3 we take ?m2 eV2 and tan2 θ23 = 1, consistent with the atmospheric neutrino 31 = 3 × 10 ?4 measurements [26], and ?m2 eV2 and tan2 θ12 = 1, consistent at present with the 21 = 10 LMA solution of the solar neutrino problem [26,27]. As concerns the third mixing angle and CP–violating phase in the lepton mixing matrix, we consider two cases. First, we take s13 = 0.2, close to the upper bound from CHOOZ [28,29,26], and δ = π/2. This set of parameters is the one that maximizes the standard CP asymmetry. Second, we take s13 = 0, in which case there is no standard CP violation in the lepton mixing. As concerns the e?ects of New Physics, we demonstrate them by taking only |?s e? | = 0. With our ?rst ?3 set of mixing parameters (maximal standard CP violation), we take |?s and δ? = 0. e? | = 10 ?4 With our second set of mixing parameters (zero standard CP violation), we take |?s e? | = 10 and δ? = π/2. Our choice of CP–violating phases can be easily understood on the basis of ? Eq. (4.8): in the large s13 limit, the CP asymmetry depends on arg[?s e? /(Ue3 U?3 )] = δ? ? δ , ? while in the small s13 limit it depends on arg[?s e? /(Ue2 U?2 )] = δ? . We use the full expression 6

for the transition probabilities that is presented in Appendix A. Consequently, the only approximation that we make is that we omit e?ects of O(|?|2 ). The results of this calculation are presented in Fig. 2. The left panels correspond to the ?rst case (maximal standard CP violation) and the right ones to the second (zero standard CP violation). For each case we present, as a function of the distance between the source SM and the detector, P+ (dotted line), P? and ASM CP (dashed lines in, respectively, upper and NP NP lower panels), and P? and ACP (solid lines in, respectively, upper and lower panels). We learn a few interesting facts: (i) The New Physics contribution to CP violation can dominate over even the maximal standard CP violation for values of |?| as small as 10?4 . This is particularly valid for distances shorter than 1000 km. (ii) The approximations that lead to Eqs. (4.7) and (4.8) are good for L < ? 5000 km. NP (iii) As anticipated from our approximate expressions, for short enough distances, P? grows linearly with distances and ANP CP is strongly enhanced at short distances. (iv) In the large s13 case, the new CP violation is sensitive mainly to the phase di?erence δ ? δ? and is almost independent of the solar neutrino parameters. (v) In the very small s13 limit, the new CP violation is proportional to sin δ? . The rate NP P? is suppressed by the solar neutrino mass di?erence and mixing angle.
V. THE TRANSITION PROBABILITY IN MATTER

Since long–baseline experiments involve the propagation of neutrinos in the matter of Earth, it is important to understand matter e?ects on our results. For our purposes, it is su?cient to study the case √ of constant matter density. Then the matter contribution to the e?ective νe mass, A = 2GF Ne , is constant. One obtains the transition probability in matter by replacing the mass-squared di?erences m ?ij and mixing angles Uαi with their e?ective values in matter, ?m ij and Uαi . The full m m expression for Pe? in matter can then be written in terms of xij and Uαi by a straightforward modi?cation of the vacuum probability given in Appendix A. To understand the matter e?ects it is, however, more useful to take into account the smallness of |Ue3 | and x12 . We SM will expand the transition probability in these parameters to second order for Pe? and to NP ?rst order for Pe? . For the Standard Model case, we obtain:
SM Pe? =4

?21 2 2 AL ?31 |Ue2 U?2 |2 + 4 sin A 2 B ?31 AL BL ?21 sin sin +8 A B 2 2

2

sin2

BL |Ue3 U?3 |2 2 (5.1)

? ? ? ? cos x31 Re[Ue 3 U?3 Ue2 U?2 ] ? sin x31 I m[Ue3 U?3 Ue2 U?2 ] ,

where B = ?31 ? A. (5.2)

Again, the ?rst term is the full result for two generations, and the second is the full result for the case of ?21 = 0. The last term violates CP. In the limit A = 0, Eq. (3.4) is reproduced. 7

Note that our de?nition of B is such that B changes sign according to whether ?31 is larger or smaller than A. This is di?erent from the usual convention where B = |?31 ? A|. The Standard Model results are an even function of B and either de?nition can be used. But for the New Physics results given below, the choice of convention is important. For the New Physics contribution we ?nd:
NP Pe? =4

?21 AL ? d? s 2 s ? sin2 Re Ue 2 U?2 ??e ? ?e? (1 ? 2|U?3 | ) + 2?eτ U?3 Uτ 3 A 2 BL ?31 ? d? s 2 s ? sin2 Re Ue ?4 3 U?3 ??e + ?e? (1 ? 2|U?3 | ) ? 2?eτ U?3 Uτ 3 B 2 ?21 ? ?d s ?2 sin(AL)I m Ue 2 U?2 (??e + ?e? ) A ?31 ? ?d s sin(BL)I m Ue ?2 3 U?3 (??e + ?e? ) B BL AL sin cos x31 ? 8 sin 2 2 ?31 ? s 2 s ? Re Ue 3 U?3 ?e? (1 ? |U?3 | ) ? ?eτ U?3 Uτ 3 B ?21 ? s 2 s ? ? Re Ue 2 U?2 ?e? |U?3 | + ?eτ U?3 Uτ 3 A BL AL sin sin x31 + 8 sin 2 2 ?31 ? s 2 s ? I m Ue 3 U?3 ?e? (1 ? |U?3 | ) ? ?eτ U?3 Uτ 3 B ?21 ? s 2 s ? ? I m Ue . 2 U?2 ?e? |U?3 | + ?eτ U?3 Uτ 3 A

(5.3)

Unlike the case of vacuum oscillation, P? will get contributions from both CP–violating terms (proportional to the imaginary parts of various combinations of parameters) as CP conserving terms (proportional to the real parts). Note that, in addition to the e?ects of new neutrino interactions in the source and in the detector, there could be other, independent e?ects due to new neutrino interactions with matter during their propagation [30–32]. Such e?ects have been studied in the context of solar and atmospheric neutrinos (see e.g. Refs. [33–35]) but we neglect them here.
VI. CP VIOLATION IN MATTER OSCILLATIONS

Since matter in Earth is not CP symmetric, there will be contributions to ACP even in the case when there is no CP violation. It is our purpose in this section to evaluate these contributions and, in particular, the fake asymmetry that is related to the real part of ?. m We denote the matter–related contribution to P? by P? ≡ P? (A) ? P? (A = 0). Since the leading contributions to P+ are the same as in the vacuum case [Eq. (4.4)], we can similarly m de?ne the matter–related contribution to ACP : Am CP ≡ P? /P+ . Note that in the evaluation s,d of Pe ?? ? from the expressions that we found for Pe? we need not only to replace Uαi and ?αβ with their complex conjugates, but also A with ?A. 8

For the Standard Model, we obtain from Eq. (5.1), in the small x31 and large s13 limits,
m SM (P ? ) =

16 4 A |Ue3 U?3 |2 . x31 3 ?31

(6.1)

In the small s13 limit (x21 /x31 ? |(Ue3 U?3 )/(Ue2 U?2 )|) the Standard Model e?ect is unobservably small, and we do not consider it here. Taking into account that [see Eq. (4.4)] 2 P+ ≈ 8x2 31 |Ue3 U?3 | , we get 2 A SM (Am = x2 . CP ) 31 3 ?31 For the New Physics contribution, we obtain from Eq. (5.3), in the small x31 limit,
m NP (P ? ) = A ? d? s 8x2 31 ?31 Re[Ue3 U?3 (??e ? ?e? )] large s13 , A ? d? s 8x2 21 ?21 Re[Ue2 U?2 (??e ? ?e? )] small s13 ,

(6.2)

(6.3)

and
NP (Am CP )

=

? ? ? ? ? ? ?

A Re ?31 A Re ?21

? s ?d ?e ??e? ? Ue3 U? 3 ? s ?d ?e ??e? ? Ue2 U?2

large s13 , (6.4) small s13 .

We would like to make a few comments regarding our results here: (i) Each of the four contributions has a di?erent dependence on the distance. In the short distance limit, we have
m SM (P ? ) ∝ L4 , SM P? ∝ L3 , m NP (P ? ) ∝ L2 , NP P? ∝ L,

(6.5)

and, equivalently,
SM (Am ∝ L2 , CP )

ASM CP ∝ L,

NP (Am ∝ L0 , CP )

ANP CP ∝ 1/L.

(6.6)

One can then distinguish between the various contributions, at least in principle. (ii) If the phases of the ?’s are of order 1, then the genuine CP asymmetry will be larger (at short distances) than the fake one. (iii) It is interesting to note that the search for CP violation in neutrino oscillations will allow us to constrain both Re(?) and I m(?). m and asymmetry Am We carried out a numerical calculation of the probabilities P± CP as a function of the distance between the source and the detector. We use again Eν = 20 GeV, ?3 ?4 ?m2 eV2 , tan2 θ23 = 1, ?m2 eV2 , tan2 θ12 = 1 and s13 = 0.2 or 0. For 31 = 3 × 10 21 = 10 ?3 the New Physics parameters, we take |?s e? | = 10 . To isolate the matter e?ects we now, however, switch o? all genuine CP violation, that is, we take δ = δ? = 0 in both cases. The results of this calculation are presented in Fig. 3. The left panels correspond to the ?rst case (large s13 ) and the right ones to the second (vanishing s13 ). For each case we present, as a function of the distance between the source and the detector, P+ (dotted line), m SM SM m NP (P ? ) and (Am (dashed lines in, respectively, upper and lower panels), and (P? ) CP ) m NP and (ACP ) (solid lines in, respectively, upper and lower panels). 9

We learn a few interesting facts: (i) The New Physics contribution to the fake CP violation can dominate over the standard contribution for values of |?| as small as 10?4 . This is particularly valid for distances shorter than 500 km. m NP (ii) As anticipated from our approximate expressions, for short enough distances (P? ) m NP grows quadratically with distances and (ACP ) is independent of the distance. m (iii) Both the standard and the new contribution to P? are suppressed by a small s13 . m NP The s13 suppression is however stronger for P+ than it is for (P? ) . Consequently, the m NP New Physics contribution to (ACP ) becomes very large for vanishing s13 . In reality, the measured P? and ACP will be a?ected by both genuine CP–violating contributions and matter-induced contributions. This situation is illustrated in Fig. 4. We SM present P+ (dotted curve), P? and ASM CP (dashed curves in, respectively, upper and lower NP NP panels), and P? and ACP (solid curves in, respectively, upper and lower panels), as a ?3 eV2 function of the distance. For the neutrino parameters, we always take ?m2 31 = 3 × 10 and tan θ23 = 1, consistent with the atmospheric neutrino data. For the other parameters, ?4 we take three cases: (a) Left panel: we take the LMA parameters (?m2 eV2 and 21 = 10 tan θ12 = 1), ‘large’ s13 = 0.2 and maximal phase δ = π/2. This choice of parameters gives ?3 maximal standard CP violation. For the New Physics parameters we take |?s and e? | = 10 δ? = 0. (The reason for the choice of phase is that the dominant contributions depend on ?6 δ ? δ? .) (b) Middle panel: we take the SMA parameters (?m2 eV2 [26,27], 21 = 5.2 × 10 ?3 2 ?4 s tan θ12 = 7.5 × 10 ), s13 = 0.2, δ = π/2, |?e? | = 10 and δ? = 0. Here the standard CP violation is unobservably small, but the standard matter e?ects are still large. (c) Right panel: we take the LMA parameters and s13 = 0. With a vanishing s13 , the total transition probability is highly suppressed as is the standard matter e?ect, and standard CP violation ?4 vanishes. For the New Physics parameters we take |?s and δ? = π/2. We take a e? | = 10 s smaller |?e? | so that our approximation will not break down. We would like to emphasize the following points: (i) Similar three cases will be the basis, in the next section, for our analysis of the sensitivity of CP–violating observables measured in neutrino factories to New Physics e?ects (see Fig. 5). (ii) With large s13 , the dependence of the New Physics e?ects (and of the standard matter-induced e?ects) on the solar neutrino parameters is very weak. (iii) A small or even vanishing s13 will suppress all the rates and will introduce a strong dependence on the solar neutrino parameters. The New Physics contributions to ACP will be, however, only little a?ected because both the standard CP conserving rate and the New Physics CP–violating rate are suppressed in the same way. (iv) With large s13 , the New Physics CP–violating e?ects are dominated by the combination δ ? δ? . With small (but not vanishing) s13 , the dependence is on both δ ? δ? and δ? . ?3 (v) For distances shorter than 800 km, the e?ects of |?| > ? 10 are always dominant. For distances shorter than 300 km, the New Physics dominates even for |?| ? 10?4 .

10

VII. LONG–BASELINE EXPERIMENTS

We would like to quantify the sensitivity of a neutrino factory to the CP–violating e?ects from new neutrino interactions. For this purpose, we consider the measurement of the following integrated asymmetry [5]: ACP = N [?? ]/N0 [e? ]|+ ? N [?+ ]/N0 [e+ ]|? . N [?? ]/N0 [e? ]|+ + N [?+ ]/N0 [e+ ]|? (7.1)

Here N [?? ]/N0 [e? ]|+ refers to an oscillation experiment that has ?+ decay as its production process: N [?? ] is the measured number of wrong–sign muons while N0 [e? ] is the expected number of νe CC interaction events (in the absence of oscillations). Similarly, N [?+ ]/N0 [e+ ]|? refers to an oscillation experiment that has ?? decay as its production process: N [?+ ] is the measured number of wrong–sign muons while N0 [e+ ] is the expected number of νe CC interaction events (again, in the absence of oscillations). The measured number of wrong–sign muon events can be expressed as follows: N [?? ]|+ = N? NT E? 2 πm2 ? L dEν fν (Eν )σCC (Eν )Pe? (Eν ), (7.2)

where NT is the number of protons in the target detector, N? is the number of useful muon decays, E? is the muon energy and m? is the muon mass. The function fν (Eν ) is the energy distribution of the produced neutrinos. We assume that the muons are not polarized, in which case fν (Eν ) = 12x2 (1 ? x) with x = Eν /E? . Finally, σCC (Eν ) is the neutrino–nucleon interaction cross section which, in the interesting range of energies, can be taken to be proportional to the neutrino energy: σCC = σ0 Eν with σ0 = 0.67 × 10?38 cm2 /GeV for neutrinos and σ0 = 0.34 × 10?38 cm2 /GeV for antineutrinos. The expression for N [e? ]|+ is obtained by an integral similar to Eq. (7.2), except that Pe? is replaced by 1. We de?ne ANP CP as the contribution from new physics (that is, ?-dependent) to the integrated CP asymmetry. We take into account both genuine CP–violating and matter-induced contributions. (In the limit of a real lepton mixing matrix, that is, no standard CP violation, the ?rst contributions are proportional to I m(?) and the latter to Re(?).) We de?ne ?A to be the statistical error on ACP . In order to quantify the signi?cance of the signal due to New Physics, we compute the ratio ANP CP /?A. The statistical error, ?A, scales with distance and energy as follows: ?A ? 1 N [?+ ]|? + N [?? ]|+ ∝ 1
SM P+ NCC

1 ∝√ . Eν

(7.3)

To ?nd this scaling, we took into account that the number of CC interactions scales as 3 SM 2 2 NCC ∝ Eν /L2 while, for L < ? 3000 km, P+ ∝ L /Eν . Consequently, the dependence of ?A on the distance is very weak. Given our results for ANP CP , we obtain the following scaling with distance of the signal-to-noise ratio: ANP CP /?A ∝ 1/L genuine CP–violating e?ects, const(L) matter induced e?ects. (7.4)

11

This behavior is illustrated in Fig. 5 where we display the signal-to-noise ratio, ANP CP /?A, s as a function of the distance. For simplicity, we consider only the e?ect of ?e? . The standard CP violation is presented only in the upper panel, where it corresponds to maximal ASM CP ?4 2 (LMA parameters: ?m2 = 10 eV and tan θ = 1, large s and δ = π/ 2), while the 12 13 21 ?6 middle panel has unobservably small ASM (SMA parameters: ? m2 eV2 and CP 21 = 5.2 × 10 2 ?4 SM tan θ12 = 7.5 × 10 ), and the lower panel has zero ACP (s13 = 0). As concerns the new CP violation, the dashed line corresponds to the case with maximal CP–violating phase ) and the solid line corresponds to purely matter-induced asymmetry (δ? = 0). In (δ? = π 2 our calculations we have assumed a total of 1021 useful ?? decays with energy E? = 50 GeV and a 40 kt detector. It is clear from the ?gure that the maximal sensitivity to new, CP–violating contributions to the production or detection processes will be achieved with shorter distances, while the sensitivity to CP conserving contributions through matter induced e?ects is almost independent of distance. A truly short baseline experiment can potentially probe the O(|?|2 ) CP conserving e?ects. But in this case, due to the small signal, systematic errors will dominate over the statistical ones discussed above. It is unlikely that |?| smaller than O(10?3) can be signalled in such a measurement. We next investigate the sensitivity to the size of the New Physics interaction that can be achieved by the measurement of the integrated CP asymmetry. In Fig. 6, we show the regions s NP in the [Re(?s e? ), I m(?e? )] plane that will lead to ACP /?A = 3 ( darker–shadow region) and ANP CP /?A = 1 (lighter–shadow regions) at L = 732 km, the shorter baseline discussed for an oscillation experiment at a neutrino factory. We have assumed a total of 1021 useful ?? decays with energy E? = 50 GeV and a 40 kt detector. In all panels we have δ = 0 (no ?3 standard CP violation), ?m2 eV2 and tan θ23 = 1, and the LMA parameters, 31 = 3 × 10 2 ?4 2 ?m21 = 10 eV and tan θ12 = 1. In the left panels we have s13 = 0.2 and in the right ones s13 = 0. In the upper panels I m(?s e? ) > 0, which, for our choice of parameters, results in a constructive interference between the matter-induced and CP–violating e?ects, while in the lower panels I m(?s e? ) < 0, which results in a destructive interference. In order to illustrate the expected improvement in sensitivity to the New Physics when the baseline is better optimized for this particular purpose, we plot in Fig. 7 the corresponding regions when the measurement of the integrated CP asymmetry is performed at a distance of L = 200 km. We would like to emphasize the following two points: (i) Fig. 6 shows that |?| in the range 3 × 10?5 –10?4 would lead to a “3σ ” e?ect. (ii) A shorter distance will improve the sensitivity to the new CP violation. Fig. 7 shows that, for δ = 0, in which case CP–violating e?ects are proportional to I m(?), an improvement by a factor of about 3 in the sensitivity to I m(?) is expected. In contrast, the sensitivity to Re(?) is not a?ected by the choice of baseline since the new physics contribution to the matter-induced asymmetry is independent of L. (iii) A non–vanishing standard CP–violating phase, δ = 0, together with a ‘large’ s13 , will change the interference pattern between the matter-induced and CP–violating contributions from new physics. The reason is that now some of the contributions depend on δ? ? δ , so that Re(?) and I m(?) do not correspond to matter-induced and CP–violating e?ects in any simple way. 12

VIII. PHENOMENOLOGICAL CONSTRAINTS
s s The measurements of Pe? and Pe ?? ? are sensitive to the four e?ective couplings, ?e? , ?eτ , d ?d ?e and ??τ . These dimensionless couplings represent new ?avor–changing (FC) neutrino interactions. They are subject to various phenomenological constraints. In this section, we present these bounds in order to compare them with the experimental sensitivity that we estimated in the previous section. ? The ?s → e? ν ?? ν? decay. For this process, e? coupling gives the amplitude for the ? there is no SU (2)L -related tree-level decay that involves four charged leptons. Instead, by closing the neutrino lines into a loop, the four-Fermi coupling contributes to the ? → eγ and ? → 3e decays. The question of how to extract reliable bounds from loop processes in an e?ective theory involves many subtleties. A calculation in the spirit of Ref. [36] yields very weak bounds. Instead, we quote here the bound in a speci?c full high energy model: if the e?ective ?L eL ν? ν? coupling is induced by an intermediate scalar triplet, the constraint from the ? → eγ decay reads (see, for example, [37]) ?5 |?s e? | ≤ 5 × 10 .

(8.1)

We emphasize again that the bound in (8.1) is model dependent and could be violated in models other than the one that we considered. ? ? The ?s ?τ ν? decay. The same coupling eτ coupling gives the amplitude for the ? → e ν ? + ? ? contributes also to the SU (2)L -related process τ → ? ? e . The experimental bound on the latter implies
?3 |?s eτ | ≤ 3.1 × 10 .

(8.2)

There could be SU (2)L breaking e?ects that would somewhat enhance the neutrino couplings with respect to the corresponding charged lepton couplings. These e?ects are discussed in detail in Refs. [38,39] where it is shown that they are constrained (by electroweak precision data) to be small. Since our purpose is only to get order–of–magnitude estimates of the bounds, we neglected the possible SU (2)L breaking e?ects in the derivation of (8.2). ? The ?d ?e coupling gives the amplitude for νe d → ? u. It is constrained by muon conversion [38]:
?6 |?d ?e | < ? 2.1 × 10 .

(8.3)

? ? ? The ?d ?τ coupling gives the amplitude for ντ d → ? u. It is constrained by the τ → ? ρ decay [39]: ?2 |?d ?τ | < ? 10 .

(8.4)

The bound on |?d ?τ | is the weakest that we obtain. Moreover, it is not unlikely that it is indeed the largest of the couplings since it is the only one not to involve a ?rst–generation lepton. For precisely the same reason, however, its contribution to Pe? is suppressed by an additional power of |Ue3 |, which is the reason that it is omitted in our approximate expressions. Let us also mention that there is a generic bound of O(0.1) on the purely leptonic couplings ?s αβ from universality in lepton decays and a somewhat weaker bound of O (0.2) on 13

the semi-hadronic couplings ?d αβ from universality in pion decays [39]. While universality is experimentally con?rmed to high accuracy, these bounds are rather weak because deviations from universality are O(?2 ). To summarize, we expect that all the ?’s that play a role in the transition probabilities of interest are of O(10?3 ) or smaller. In the previous section, we learnt that proposed experiments might probe these couplings down to values as small as O(10?4 ). This means that the possibility to measure new neutrino interactions through CP violation in neutrino oscillation experiments is open. Conversely, such future experiments can improve the existing bounds on FC neutrino interactions which, at present, come from rare charged lepton decays.
IX. CONCLUSIONS AND DISCUSSION

We summarize the main points of our study: (i) CP–violating observables are particularly sensitive to new physics. The reason is that the standard CP violation that comes from the lepton mixing matrix gives e?ects that are particularly suppressed by small mass di?erences and mixing angles. Some of these suppression factors do not apply to new contributions. (ii) The fact that matter e?ects contribute to CP-violating observables means that these observables are sensitive to both the CP conserving and the CP-violating contributions from New Physics. (iii) The e?ects of New Physics in the production and detection processes depend on the source–detector distance in a way that is di?erent from the standard one. One consequence of this situation is that, at least in principle, it is possible to disentangle standard and new e?ects. Another consequence is that in short distance experiments the new e?ects are enhanced. (iv) Our rough estimate is that future neutrino factories will be able to probe, through CP–violating observables, e?ects from new interactions that are up to about four orders of magnitude weaker than the weak interactions. (v) The sensitivity to New Physics e?ects is better than most of the existing model– independent bounds. We would like to mention that a similar (and, for speci?c models, even stronger) level of sensitivity may be achieved by other experiments that search for lepton ?avor violation. Particularly promising are those involving muon decay and conversion (for a recenet review, see [44]): for example, a future experiment at PSI will be sensitive to B (? → eγ ) at the 10?14 level [45], and the MECO collaboration has proposed an experiment to probe ? ? e conversion down to 5 × 10?17 , four orders of magnitude beyond present sensitivities [46]. If these experiments observe a signal, the search for related CP violation will become of particular importance. What type of new physics will be implied in case that a signal is observed? The ? couplings represent e?ective four–fermion interactions coming from the exchange of heavy particles related to New Physics. If the New Physics takes place at some high scale ΛNP, then one can set an upper bound: m2 Z < ?s,d αβ ? 2 . Λ
NP

(9.1)

14

The source of this bound is in the de?nition of ?, which is the ratio of the four–fermion operator to GF , and the fact that it is maximal when the New Physics contribution comes at tree level and the couplings are of order one. Since the expected experimental sensitivity is to |?| ≥ O(10?4 ), we learn that we can probe models with ΛNP < ? 10 T eV. (9.2)

If the New Physics contributes to the relevant processes only at the loop level, there is another suppression factor in |?| of order 161 . That would mean that such models can π2 be probed only if ΛNP < 1 T eV . Finally, if the ?avor changing nature of the interaction ? s introduces a suppression factor, e.g. |?e? | ? m? /ΛNP , that by itself would be enough to make it unobservable in near future experiments. We thus learn that CP violation in neutrino oscillation experiments will explore models with a scale that is, at most, one to two orders of magnitude above the electroweak breaking scale, and where the ?avor structure is di?erent from the Standard Model. Another point concerns the Dirac structure of the four-Fermi interaction. We did not present it explicitly in our discussion of the Gs,d NP couplings. However, it is implicitly assumed in our discussion that the Dirac structure is the same as that of the weak interactions, i.e. a (V-A)(V-A) structure. The reason for that is that the e?ects that we discuss are a consequence of interference between weak and new interactions. A di?erent Dirac structure would give strong suppression factors related to the the charged lepton masses. While our formalism would still apply, these suppression factors would make the related e?ects practically unobservable. We conclude that a signal is likely to imply new physics at a relatively low scale (up to 1–10 TeV) with new sources of ?avor (and, perhaps, CP) violation. We know of several well motivated extensions of the Standard Model that can, in principle, induce large enough couplings. In particular, we have in mind loop contributions involving sleptons and gauginos in supersymmetric models, tree contributions involving charged singlet sleptons in supersymmetric models without R-parity, and tree contributions involving a triplet scalar in left-right symmetric models. In another class of relevant models, such as the model of Ref. [40], active neutrinos mix with singlet neutrinos. (Here there can be Z -mediated contributions to the non-standard couplings, and the phenomenological constraints are di?erent [41,42].) A detailed analysis of new neutrino interactions within relevant extensions of the Standard Model is beyond the scope of this paper, but preliminary results show that large enough couplings are allowed and in some cases even predicted [43].
ACKNOWLEDGMENTS

MCGG and YN thank the school of natural sciences in the Institute for Advanced Study (Princeton), where part of this work was carried out, for the warm hospitality. MCG-G is supported by the European Union Marie-Curie fellowship HPMF-CT-2000-00516. AG is supported by Funda? c? ao Coordena? c? ao de Aperfei? coamento de Pessoal de N? ?vel Superior (CAPES). YN is supported by the Israel Science Foundation founded by the Israel Academy of Sciences and Humanities, by the United States - Israel Binational Science Foundation (BSF) and by the Minerva Foundation (Munich). This work was also supported by the 15

Spanish DGICYT under grants PB98-0693 and PB97-1261, by the Generalitat Valenciana under grant GV99-3-1-01 and by the TMR network grant ERBFMRXCT960090 of the European Union and ESF network 86. This work was also supported by the fund for the promotion of research at the Technion.

16

APPENDIX A: TRANSITION PROBABILITY IN VACUUM

Neglecting terms of O(?2 ) and with no other approximations, we obtain the following expression for Pe? : Pe? =
? 2 ? 2 4 sin2 x21 |U?2 |2 |Ue2 |2 ? Re ?s e? (Ue1 U?1 |U?2 | + Ue2 U?2 |U?1 | ) ? 2 ? 2 s ? ? ? ? + ?d ?e (U?2 Ue2 |Ue1 | + U?1 Ue1 |Ue2 | ) + ?eτ (Ue2 U?2 U?1 Uτ 1 + Ue1 U?1 U?2 Uτ 2 ) ? ? ? ? ? ? + ?d ?τ (U?2 Ue2 Ue1 Uτ 1 + U?1 Ue1 Ue2 Uτ 2 ) ? Ue2 Ue3 U?3 U?2 ? 2 ? 2 + 2 sin 2x21 I m ?s e? (Ue1 U?1 |U?2 | ? Ue2 U?2 |U?1 | ) ? 2 ? 2 s ? ? ? ? + ?d ?e (U?2 Ue2 |Ue1 | ? U?1 Ue1 |Ue2 | ) + ?eτ (Ue2 U?2 U?1 Uτ 1 ? Ue1 U?1 U?2 Uτ 2 ) ? ? ? ? ? ? + ?d ?τ (U?2 Ue2 Ue1 Uτ 1 ? U?1 Ue1 Ue2 Uτ 2 ) + Ue2 Ue3 U?3 U?2 ? 2 ? 2 + 4 sin2 x31 |U?3 |2 |Ue3 |2 ? Re ?s e? (Ue1 U?1 |U?3 | + Ue3 U?3 |U?1 | ) ? ? ? ? ? 2 ? 2 s + ?d ?e (U?3 Ue3 |Ue1 | + U?1 Ue1 |Ue3 | ) + ?eτ (Ue1 U?1 U?3 Uτ 3 + Ue3 U?3 U?1 Uτ 1 ) ? ? ? ? ? ? + ?d ?τ (U?3 Ue3 Ue1 Uτ 1 + U?1 Ue1 Ue3 Uτ 3 ) ? Ue2 Ue3 U?3 U?2 ? 2 ? 2 + 2 sin 2x31 I m ?s e? (Ue1 U?1 |U?3 | ? Ue3 U?3 |U?1 | ) ? 2 ? 2 s ? ? ? ? + ?d ?e (U?3 Ue3 |Ue1 | ? U?1 Ue1 |Ue3 | ) + ?eτ (Ue1 U?1 U?3 Uτ 3 ? Ue3 U?3 U?1 Uτ 1 ) ? ? ? ? ? ? + ?d ?τ (U?3 Ue3 Ue1 Uτ 1 ? U?1 Ue1 Ue3 Uτ 3 ) ? Ue2 Ue3 U?3 U?2 ? 2 ? 2 ? 4 sin2 x32 Re ?s e? (Ue2 U?2 |U?3 | + Ue3 U?3 |U?2 | ) ? 2 ? 2 s ? ? ? ? + ?d ?e (U?3 Ue3 |Ue2 | + U?2 Ue2 |Ue3 | ) + ?eτ (Ue2 U?2 U?3 Uτ 3 + Ue3 U?3 U?2 Uτ 2 ) ? ? ? ? ? ? + ?d ?τ (U?3 Ue3 Ue2 Uτ 2 + U?2 Ue2 Ue3 Uτ 3 ) + Ue2 Ue3 U?3 U?2 ? 2 ? 2 + 2 sin 2x32 I m ?s e? (Ue2 U?2 |U?3 | ? Ue3 U?3 |U?2 | ) ? 2 ? 2 s ? ? ? ? + ?d ?e (U?3 Ue3 |Ue2 | ? U?2 Ue2 |Ue3 | ) + ?eτ (Ue2 U?2 U?3 Uτ 3 ? Ue3 U?3 U?2 Uτ 2 ) ? ? ? ? ? ? + ?d ?τ (U?3 Ue3 Ue2 Uτ 2 ? U?2 Ue2 Ue3 Uτ 3 ) + Ue2 Ue3 U?3 U?2 .

(A1)

17

REFERENCES
[1] A. De R? ujula, M. B. Gavela and P. Hernandez, Nucl. Phys. B 547, 21 (1999) [hepph/9811390]. [2] V. Barger, S. Geer and K. Whisnant, Phys. Rev. D 61, 053004 (2000) [hep-ph/9906487]. [3] M. Tanimoto, Phys. Lett. B 462, 115 (1999) [hep-ph/9906516]. [4] H. Minakata and S. Watanabe, Phys. Lett. B 468, 256 (1999) [hep-ph/9906530]. [5] A. Donini, M. B. Gavela, P. Hernandez and S. Rigolin, Nucl. Phys. B 574, 23 (2000) [hep-ph/9909254]. [6] A. Romanino, Nucl. Phys. B 574, 675 (2000) [hep-ph/9909425]. [7] M. Koike and J. Sato, Phys. Rev. D 61, 073012 (2000) [Erratum-ibid. D 62, 079903 (2000)] [hep-ph/9909469]; Phys. Rev. D 62, 073006 (2000) [hep-ph/9911258]. [8] M. Freund, M. Lindner, S. T. Petcov and A. Romanino, Nucl. Phys. B 578, 27 (2000) [hep-ph/9912457]. [9] A. Cervera, A. Donini, M. B. Gavela, J. J. Gomez Cadenas, P. Hernandez, O. Mena and S. Rigolin, Nucl. Phys. B 579, 17 (2000) [Erratum-ibid. B 593, 17 (2000)] [hepph/0002108]. [10] I. Mocioiu and R. Shrock, Phys. Rev. D 62, 053017 (2000) [hep-ph/0002149]. [11] V. Barger, S. Geer, R. Raja and K. Whisnant, Phys. Rev. D 62, 073002 (2000) [hepph/0003184]; Phys. Rev. D 63, 033002 (2001) [hep-ph/0007181]; hep-ph/0012017. [12] H. Minakata and H. Nunokawa, Phys. Lett. B 495, 369 (2000) [hep-ph/0004114]. [13] K. Kimura and A. Takamura, hep-ph/0008024. [14] H. Yokomakura, K. Kimura and A. Takamura, Phys. Lett. B 496, 175 (2000) [hepph/0009141]. [15] A. Bueno, M. Campanelli, M. Laveder, J. Rico and A. Rubbia, hep-ph/0010308. [16] S. J. Parke and T. J. Weiler, Phys. Lett. B 501, 106 (2001) [hep-ph/0011247]. [17] P. Lipari, hep-ph/0102046. [18] J. Burguet-Castell, M. B. Gavela, J. J. Gomez-Cadenas, P. Hernandez and O. Mena, hep-ph/0103258. [19] M. Freund, hep-ph/0103300. [20] E. K. Akhmedov, P. Huber, M. Lindner and T. Ohlsson, hep-ph/0105029. [21] M. Freund, P. Huber and M. Lindner, hep-ph/0105071. [22] J. Pinney and O. Yasuda, hep-ph/0105087. [23] Z. Maki, M. Nakagawa and S. Sakata, Prog. Theor. Phys. 28, 870 (1962). [24] M. Kobayashi and T. Maskawa, Prog. Theor. Phys. 49, 652 (1973). [25] Y. Grossman, Phys. Lett. B 359, 141 (1995) [hep-ph/9507344]. [26] M. C. Gonzalez-Garcia, M. Maltoni, C. Pena-Garay and J. W. Valle, Phys. Rev. D 63, 033005 (2001) [hep-ph/0009350]. [27] M. C. Gonzalez-Garcia and C. Pena-Garay, Nucl. Phys. Proc. Suppl. 91, 80 (2000) [hep-ph/0009041]. [28] M. Apollonio et al. [CHOOZ Collaboration], Phys. Lett. B 420, 397 (1998) [hepex/9711002]; Phys. Lett. B 466, 415 (1999) [hep-ex/9907037]. [29] F. Boehm et al. [Palo Verde Experiment], Phys. Rev. Lett. 84, 3764 (2000) [hepex/9912050]; Phys. Rev. D 62, 072002 (2000) [hep-ex/0003022]. [30] J. W. Valle, Phys. Lett. B 199, 432 (1987). [31] E. Roulet, Phys. Rev. D 44, 935 (1991). 18

[32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44]

[45] [46]

M. M. Guzzo, A. Masiero and S. T. Petcov, Phys. Lett. B 260, 154 (1991). P. I. Krastev and J. N. Bahcall, hep-ph/9703267. S. Bergmann, Nucl. Phys. B 515, 363 (1998) [hep-ph/9707398]. M. C. Gonzalez-Garcia et al., Phys. Rev. Lett. 82, 3202 (1999) [hep-ph/9809531]. C. P. Burgess and D. London, Phys. Rev. D 48, 4337 (1993) [hep-ph/9203216]. F. Cuypers and S. Davidson, Eur. Phys. J. C 2, 503 (1998) [hep-ph/9609487]. S. Bergmann and Y. Grossman, Phys. Rev. D 59, 093005 (1999) [hep-ph/9809524]. S. Bergmann, Y. Grossman and D. M. Pierce, Phys. Rev. D 61, 053005 (2000) [hepph/9909390]. Y. Grossman and M. Neubert, Phys. Lett. B 474, 361 (2000) [hep-ph/9912408]. S. Bergmann and A. Kagan, Nucl. Phys. B 538, 368 (1999) [hep-ph/9803305]. R. Kitano, Phys. Lett. B 481, 39 (2000) [hep-ph/0002279]. M.C. Gonzalez-Garcia, Y. Grossman, A. Gusso and Y. Nir, work in progress. Y. Kuno and Y. Okada, Rev. Mod. Phys. 73, 151 (2001) [hep-ph/9909265]; See also contributions in Proceedings of the Workshop on Physics at the First Muon Collider and at the Front End of a Muon Collider, eds. S. Geer and R. Raja (American Institute of Physics, Woodbury, New York, 1998). L. M. Barkov et al., research proposal R–99–05, “Search for ?+ → e+ γ at 10?14 ,” 1999. MECO and KOPIO Collaborations, “A Proposal to the NSF to Construct the MECO and KOPIO Experiments,” 1999, http://meco.ps.uci.edu/RSVP.html; W. Molzon, in Proceedings of Lepton and Baryon Number Violation in Particle Physics, Astrophysics and Cosmology, eds. H. V. Klapdor-Kleingrothaus and I. V. Krivosheina (Institute of Physics Publishing, Bristol and Philadelphia, 1999), p. 586.

19

δ δ’ ε
d* Ue3U* ε ?e ?3

ε e?
δ ε

s

* Ue1U? 1

Ue2U* ?2

FIG. 1. The neutrino parameters that dominate Pe? in the complex plane. We show the relevant unitarity triangle, which is the geometrical presentation of the relation ? + U U ? + U U ? = 0, and the two parameters that describe the New Physics in the Ue1 U? e2 ?2 e3 ?3 1 s ? production, ?e? , and in the detector, ?d ?e . The three independent phases de?ned in the text, δ , δ? ? on the real axis. ′ and δ? , are shown explicitly. The standard convention puts Ue1 U? 1

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FIG. 2. Transition probabilities and CP asymmetries in vacuum as a function of the distance. SM (dotted), P SM (dashed) and P NP (solid). In the In the upper panels the curves correspond to P+ ? ? SM (dashed). In the left panels, s lower panels the curves correspond to ANP (solid) and A 13 = 0.2, CP CP ? 4 ? 3 s s δ = π/2, |?e? | = 10 and δ? = 0. In the right panels, s13 = 0, |?e? | = 10 and δ? = π/2. In all ?3 eV2 , tan2 θ = 1, ?m2 = 10?4 eV2 and tan2 θ curves Eν = 20 GeV, ?m2 12 = 1. 23 21 13 = 3 × 10

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FIG. 3. Transition probabilities and fake CP asymmetries in matter as a function of the distance. All CP–violating phases are set to zero. In the upper panels the curves correspond to SM (dotted), (P m )SM (dashed) and (P m )NP (solid). In the lower panels the curves correspond P+ ? ? NP (solid) and (Am )SM (dashed). In the left panels s to (Am 13 = 0.2, and in the right panels CP ) CP ?4 eV2 , ?3 eV2 , tan2 θ 2 = 3 × 10 s13 = 0. In all curves Eν = 20 GeV, ?m2 23 = 1, ?m21 = 10 31 ?3 tan2 θ12 = 1, δ = δ? = 0 and |?s e? | = 10 .

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FIG. 4. Transition probabilities and fake CP asymmetries in matter as a function of the SM (dotted), P SM (dashed) and P NP distance. In the upper panels the curves correspond to P+ ? ? NP (solid). In the lower panels the curves correspond to ACP (solid) and ASM (dashed). In all curves CP ?4 eV2 , ?3 eV2 and tan2 θ 2 Eν = 20 GeV, ?m2 23 = 1. In the left panels ?m21 = 10 31 = 3 × 10 s ? 3 2 tan θ12 = 1, s13 = 0.2, δ = π/2, |?e? | = 10 and δ? = 0. In the middle panels ?m21 = 5.2 × 10?6 ?3 and δ = 0. In the right panels eV2 , tan2 θ12 = 7.5 × 10?4 , s13 = 0.2, δ = π/2, |?s ? e? | = 10 ? 4 2 s ? 4 2 ?m21 = 10 eV , tan θ12 = 1, s13 = 0, |?e? | = 10 and δ? = π/2.

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FIG. 5. The signal-to-noise ratio, ANP CP /?A, as a function of the distance L. We considered the following parameters for the experiment: E? = 50 GeV, 1021 ?? decays and a 40 kt detector, and ?3 eV2 , tan θ the following neutrino parameters: δ = 0, ?m2 23 = 1. In the upper and 31 = 3 × 10 lower (middle) panels we use the LMA (SMA) parameters. In the upper two (lower) panels we use ?3 and δ = 0 or π/2. In the upper panel, the s13 = 0.2(0). For the New Physics we take |?s ? e? | = 10 SM dotted curve gives the SM matter-subtracted asymmetry ACP (δ = π/2) ? ASM CP (δ = 0).

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s NP FIG. 6. Regions in the plane of [Re(?s e? ), I m(?e? )] that give ACP /?A = 3 (darker shadow) and 1 (light shadow). For the experiment, we take L = 732 km, E? = 50 GeV, 1021 ?? decays and a ?3 eV2 , tan2 θ 40 kt detector. For the neutrino parameters, we take δ = 0, ?m2 23 = 1, 31 = 3 × 10 ? 4 2 2 2 ?m21 = 10 eV and tan θ12 = 1. In the left (right) panels we have s13 = 0.2(0).

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s NP FIG. 7. Regions in the plane of [Re(?s e? ), I m(?e? )] that give ACP /?A = 3 (darker shadow) and 1 (light shadow). For the experiment, we take L = 200 km, E? = 50 GeV, 1021 ?? decays and a ?3 eV2 , tan2 θ 40 kt detector. For the neutrino parameters, we take δ = 0, ?m2 23 = 1, 31 = 3 × 10 ? 4 2 2 2 ?m21 = 10 eV and tan θ12 = 1. In the left (right) panels we have s13 = 0.2(0). Note that the scales in the right panels are di?erent from the left panels and from Fig. 6.

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